2001 Mitsubishi ES-V6. The DTC is P0304 - Cylinder #4 misfire. The mechanic have replaced the Cap/Rotor, Spark Plug Wire, Spark Plug, Valve cover gasket (leaking oil into the plugs). Included in the repair, I had the mechanic replace timing belt, belt tensioner, water pump, tensioner idler, cam and crank seals, accessory belts. He also performed all these test and they are good -- vacuum leaks test, compression test, leak down test, and fuel injectors. When the misfire occurs, the vacuum reading rapidly fluctuates.
Not familiar with this particular model but swap injector wih number 1 and see if fault transfers over as injector coil may fail intermittantly.only other thing left is switched wire from ecm to number 4 injector or ecm internal fault ?
valve spring or worn cam lobe will cause a miss but if providing enough valve seat pressure may still pass a compression test.however from your description i thought the problem was intermittant ruling these out. an egr valve fault will perhaps cause a rough running engine and vac fluctuations but not specific to one cylinder.connecting an ossilascope in parallel to number 4 injector plug will show any fluctuation or loss of injector pulse to that cylinder. your mechanic should of checked for oiling of the sparkplug due to oil ring failure
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Look for weak valve springs.
when you use a vacuum gauge.when your needle fluctuation is rapid. you can have weak valve springs.weak valve.intake manifold leak.slight head gasket leak.you have something going on in cylinder tell me what your compression was.and was your cylinder leak down test 20% or more.and what was compression 100 psi or less.
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Yes the injector were swapped and the problem remains in cylinder #4. I did not check the ecm yet. I have also opened the valve cover and checked the all the springs and pistons. They seem to be in excellent condition. Is it possible to have weak springs when the compression is good?
changed wires and spark plugs
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